MRI apparatuses are medical diagnostic imaging apparatuses that induce a nuclear magnetic resonance phenomenon in atomic nuclei in an arbitrary cross-section of a subject, and obtain a tomographic image of the section from generated magnetic resonance signals. The apparatuses transmit a radio frequency wave (henceforth referred to as radio frequency wave or RF), which is a kind of electromagnetic wave, to a subject to excite nuclear spins in the subject, then receive magnetic resonance signals generated by the nuclear spins, and construct images of the subject. The transmission is performed with an RF transmission coil, and the reception is performed with an RF reception coil.
In recent years, in order to improve SNR (signal to noise ratio) of images, static magnetic field intensity used in MRI apparatuses has tended to increase, and high magnetic field MRI apparatuses using a static magnetic field intensity of about 3 T (tesla) (3T MRI apparatuses) have begun to spread. However, use of higher static magnetic field intensity more easily invites unevenness of obtained images. This is because a higher frequency of RF is used for inducing the magnetic resonance phenomenon with use of higher magnetic field intensity. In 3T MRI apparatuses, RF having a frequency of 128 MHz is used. The wavelength of this RF in living bodies is about 30 cm, which is in substantially the same scale as a section of the abdominal part, and thus phase of RF changes in the living bodies. Therefore, irradiated RF distribution and spatial distribution of a rotating magnetic field (henceforth referred to as B1) that is generated by the RF and induces the magnetic resonance phenomenon become inhomogeneous, and unevenness of the image is generated. Under such a current circumstance, there is desired a technique for reducing inhomogeneity in distribution of the rotating magnetic field B1 at the time of the RF irradiation performed in high magnetic field MRI apparatuses, in order to improve image quality.
As a method for reducing the inhomogeneity of B1 distribution, use of a “dielectric pad” has been proposed (refer to, for example, Non-patent document 1). That is, a pad having a certain dielectric constant is placed on an imaging part, such as the abdominal part, to change the B1 distribution in the abdominal part and thereby attain an effect of shifting position of a part of low B1 intensity.
Research has also been done on putting a coupling coil on the abdominal part (refer to Patent document 1 and Non-patent document 2). That is, by placing a coupling coil near a part of low B1 intensity in a section of the abdominal part, an effect of increasing the B1 intensity is obtained.